The present invention relates to a liquid level detection device for detecting the liquid level in a fuel tank in a vehicle for example, and a method of manufacturing a conductor electrode used therefor, and more particularly to an improvement to a contact portion provided at a sliding body or a conductor electrode provided at an insulating substrate.
There has been a well-known liquid level detection device including a moving element including a float moving up and down according to the liquid level in a fuel tank, a contact portion provided at a sliding body sliding in synchronization with the movement of the moving element, and an insulating substrate provided with a conductor electrode having a prescribed arrangement pattern depending upon the sliding track of the contact portion, the upward and downward movement of the liquid level is transmitted to the sliding body through the moving element, the contact portion of the sliding body slides on the conductor electrode into contact with the conductor electrode, and the resistance value changes depending upon the position of the conductor electrode in contact, so that the liquid level in the fuel tank is detected.
In such a liquid level detection device, for the material of the contact portion, a AgPd (silver-palladium) alloy, a AgCu (silver-copper) alloy, a AgNi (silver-nickel) alloy or the like is generally used. A conductor electrode 4 is for example made of a mixture of AgPd (silver-palladium) powder and glass as disclosed by Japanese Patent Publication No. 4-1682, and it has been known that the conductor electrode of such mixture is obtained by mixing Ag (silver) powder, palladium (Pd) powder and glass powder into paste, and printing the paste on an insulating substrate, followed by drying and sinter. Although Ag (silver) has small electrical resistance and good electrical conductivity, when used in fuel, it could deteriorate and corrode because of the sulfur component, water, alcohol or the like in the fuel, which leads to a conduction defect. Therefore, for the contact portion, an alloy is formed with the addition of an anti-degradation, anti-corrosion material such as Pd (palladium), Cu (copper), and Ni (nickel), while for the conductor electrode, a mixture is formed with the addition of an anti-degradation, anti-corrosion material of Pd (palladium) as countermeasure against degradation and corrosion.
Note however that such anti-degradation and anti-corrosion measure is not sufficient in the contact portion or conductor electrode, and if the fuel is of gasoline, the sulfur therein sulfurizes the Ag (silver) component, which sometimes causes a contact defect. Therefore, there has been a strong demand for a contact portion or conductor electrode which withstands the degradation by sulfurization or the like. Note that in this point Japanese Utility Model Publication No. 4-1682 or Japanese Utility Model Laying-Open No. 6-40821 discloses the arrangement of covering the top of the conductor electrode of a Ag (silver) alloy with a conductive protection film for degradation protection in order to prevent the alloy including Ag (silver) from degrading, but the additional protection layer inevitably and disadvantageously increases the cost and man-hours.
The present invention therefore provides a liquid level detection device having a contact portion or conductor electrode which withstands degradation or corrosion by sulfurization or the like, or a method of manufacturing a conductor electrode used therefor. The invention specifically provides a liquid level detection device having a contact portion or conductor electrode which withstands degradation or corrosion or a method of manufacturing a conductor electrode used therefor while restraining the increase in the cost or man-hours.
The liquid level detection device according to the present invention includes a moving element operating according to the liquid level in a fuel tank, a contact portion provided at a sliding body operating in synchronization with the movement of the moving element, and an insulating substrate provided with a conductor electrode which the contact portion slides on, the contact portion is formed of an alloy free from Ag (silver), and therefore the contact portion is less degraded by sulfurization or the like, so that the durability of the contact portion may be improved. Such an alloy without Ag (silver) includes any alloy having an anti-degradation characteristic in fuel, and for this kind of alloys, an alloy including CuNi (copper-nickel) as a main constituent, or an alloy including CuNiZn (copper-nickel-zinc) as a main constituent or an alloy including this is preferable because it is relatively inexpensive and yet has high degradation resistance. The use of an alloy including CuNiZn (copper-nickel-zinc) as a main constituent is particularly advantageous in many respects because the Zn (zinc) can prevent oxidation and has good resistance against alcohol and water included in the fuel.
Also in this case, if the conductor electrode side is covered with a conductive protection film having corrosion resistance, the anti-degradation characteristic not only on the contact portion side but also on the conductor electrode side can be improved, and such a conductive protection film includes any film as long as it can protect the conductor electrode, and may be formed for example by Ni (nickel) plating.
A method of manufacturing a conductor electrode for use in a liquid level detection device according to the present invention includes the step of printing paste formed by mixing powder including AgPd (silver-palladium) alloy powder and glass powder on an insulating substrate, and the step of sinter said paste printed on said insulating substrate, so that the resulting conductor electrode may have high degradation resistance, and an additional conductive protection film which has been required in the conventional technique is not necessary, which can restrain increase in the cost and man-hours.
The liquid level detection device according to the present invention includes a moving element operating according to the liquid level in a fuel tank, a contact portion provided at a sliding body operating in synchronization with the movement of the moving element, and an insulating substrate provided with a conductor electrode which the contact portion slides on, the conductor electrode made of a mixture of the AgPd (silver-palladium) alloy powder and glass is less susceptible to degradation such as sulfurization, the durability of the conductor electrode can be improved, an additional conductive protection film which has been necessary in the conventional technique is not necessary, and therefore the cost and man-hours can be prevented from increasing. As the AgPd (silver-palladium) alloy powder, only an AgPd (silver-palladium) alloy may be used, but a powder component of another material may be added or supplemented to powder including this alloy as a main constituent.
In this case, if the ratio of Pd (palladium) in the alloy including AgPd (silver-palladium) as a main constituent is set to approximately 40% by weight or higher, degradation resistance higher than that of an alloy including approximately 40% by weight or less of Pd (palladium) results.
Also in this case, if the contact portion side is made of an alloy including copper and nickel as main constituents or an alloy including copper, nickel and zinc as main constituents, the anti-degradation characteristic on the conductor electrode side may be improved as well, which improves the durability.
Furthermore, according to the present invention, since the hardness of the contact portion is lower than that of said conductor electrode, abrasion at the conductor electrode having a small thickness may be prevented, and high durability results. Also in this case, since the contact portion is formed in a contact member separately attached to the sliding body, the thickness of the contact portion does not depend on the thickness of the sliding body as compared to the conventional case where the conductor plate itself serving as a sliding body is partially deformed at a prescribed position to form a contact portion, so that the thickness of the contact portion may be increased, and a contact portion capable of withstanding abrasion caused on the conductor electrode for its low hardness can be provided.
In addition, according to the present invention, there are provided a moving element operating according to the liquid level in a fuel tank, a contact portion provided at a sliding body operating in synchronization with the movement of the moving element, and an insulating substrate provided with a conductor electrode which the contact portion slides on, the contact portion is made of an alloy of a first conductive material other than silver and a second conductive material other than Pd (palladium) having degradation or corrosion resistance in fuel, and therefore the anti-degradation and anti-corrosion characteristics may be improved inexpensively. As the first conductive material, Cu (copper) is preferable for its small electrical resistance, good electrical conductivity, and low price. As the second conductive material, Ni (nickel) is preferable for its high anti-degradation and anti-corrosion characteristics and low price. Since said contact portion includes Zn (zinc), the anti-oxidation characteristic can be improved, and resistance against alcohol and water in fuel may be provided.